U.S. patent number 5,879,406 [Application Number 08/893,278] was granted by the patent office on 1999-03-09 for artificial joint bioprosthesis for mitigation of wear.
This patent grant is currently assigned to Saint-Gobain Industrial Ceramics, Inc.. Invention is credited to Edward Lilley.
United States Patent |
5,879,406 |
Lilley |
March 9, 1999 |
Artificial joint bioprosthesis for mitigation of wear
Abstract
This invention relates to an acetabular cup having a
substantially hemispherical ceramic concave surface comprising at
least one debris reservoir thereon having a width of between 0.010
mm and 2 mm, and also to a ceramic hip joint prosthesis head whose
outer surface comprises a similar debris reservoir.
Inventors: |
Lilley; Edward (Shrewsbury,
MA) |
Assignee: |
Saint-Gobain Industrial Ceramics,
Inc. (Worcester, MA)
|
Family
ID: |
25401318 |
Appl.
No.: |
08/893,278 |
Filed: |
July 15, 1997 |
Current U.S.
Class: |
623/22.15 |
Current CPC
Class: |
A61F
2/30771 (20130101); A61F 2/32 (20130101); A61F
2310/00023 (20130101); A61F 2002/365 (20130101); A61F
2002/30822 (20130101); A61F 2002/3437 (20130101); A61F
2002/30593 (20130101); A61F 2002/30937 (20130101); A61F
2002/3435 (20130101); A61F 2002/30652 (20130101); A61F
2220/0033 (20130101); A61F 2002/3621 (20130101); A61F
2002/3495 (20130101); A61F 2002/30673 (20130101); A61F
2002/30332 (20130101); A61F 2/36 (20130101); A61F
2310/00239 (20130101); A61F 2002/30838 (20130101); A61F
2002/30827 (20130101); A61F 2002/30154 (20130101); A61F
2230/0021 (20130101); A61F 2002/30807 (20130101); A61F
2310/00029 (20130101); A61F 2002/30968 (20130101); A61F
2002/30683 (20130101); A61F 2002/3613 (20130101); A61F
2310/00203 (20130101) |
Current International
Class: |
A61F
2/30 (20060101); A61F 2/32 (20060101); A61F
2/00 (20060101); A61F 2/36 (20060101); A61F
2/34 (20060101); A61F 002/32 () |
Field of
Search: |
;623/18,20,21,22,23 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
346294 |
|
Dec 1989 |
|
EP |
|
2242065 |
|
Apr 1974 |
|
FR |
|
2 537 868 |
|
Jun 1984 |
|
FR |
|
1 127 584 |
|
Dec 1984 |
|
SU |
|
2191402 |
|
Dec 1987 |
|
GB |
|
Other References
Tanchuling Jr., Antonio et al, "Does Reverse Hybrid Total Hip
Arthroplasty Cause Lower Wear Rate and Osteolysis?", AAOS, 1997.
.
Khalily, C. et al, "Effect of Locking Mechanism on Particle and
Fluid Migration Through Modular Acetabular Components", 64th Annual
Meeting of the American Academy of Orthopaedic Surgeons, Feb.
13-17, 1997. .
Reflection I & FSO Porous-Coated Acetabular Component Surgical
Technique, Technique Described by John M. Cuckler, M.D., pp.
1-16..
|
Primary Examiner: Jones; Mary Beth
Attorney, Agent or Firm: DiMauro; Thomas M.
Claims
I claim:
1. A ceramic hip joint prosthesis head comprising:
a) a substantially spherical outer surface having a diameter of
between 15 mm and 50 mm, and
b) a tapered recess extendng inward from the outer surface, the
recess having an outer diameter of at least 4 mm and a depth of at
least 6 mm
wherein the outer surface comprises at least on debris reservoir
thereon having a width of between 0.010 mm and 2 mm.
2. The head of claim 1 wherein the ceramic is selected from the
group consisting of alumina and zirconia, and mixtures thereof.
3. The head of claim 2 wherein the ceramic is zirconia.
4. The head of claim 1 wherein the debris reservoir has a width of
between about 0.010 mm and 0.100 mm, and a depth of between about
0.01 mm and 0.1 mm.
5. The head of claim 1 having a centerpoint, wherein the outer
surface of the head defines an apex located substantially opposite
the recess and an axis connecting the apex and the centerpoint, and
wherein at least one debris reservoir is provided on the outer
surface substantially concentrically about the apex, the reservoir
having a radius which defines a line between the centerpoint and
the radius of the debris reservoir, the line and the axis defining
an intersection at the centerpoint, wherein the intersection of the
axis and the line defines an angle .alpha. of between 10 and 60
degrees.
6. The head of claim 1 wherein the debris reservoir is located
substantially opposite the recess.
7. The head of claim 1 having a centerpoint, wherein the outer
surface of the head defines an apex located substantially opposite
the recess and an axis connecting the apex and the centerpoint, and
wherein the at least one debris reservoir is provided on the outer
surface substantially concentrically about a midpoint which defines
a line between the centerpoint and the midpoint, the line and the
axis defining an intersection at the centerpoint, wherein the
intersection of the axis and the line defines an angle .tau. of
between 10 and 60 degrees.
8. The head of claim 7 wherein the radius of the debris reservoir
defines an angle .theta. of no more than about 20 degrees.
9. The head of claim 7 wherein the intersection of the axis and the
line defines an angle .tau. of between 30 and 45 degrees.
10. The head of claim 1 further comprising i) an inner void and ii)
a channel which provides fluid connection between the inner void
and the at least one debris reservoir.
11. An acetabular cup having a substantially hemispherical ceramic
concave surface having a maximum diameter of between 15 mm and 50
mm, and further comprising at least one debris reservoir on the
concave surface having a width of between 0.010 mm and 2 mm.
12. The cup of claim 11 wherein the ceramic is selected from the
group consisting of alumina and zirconia.
13. The cup of claim 11 wherein the debris reservoir has a width of
between about 0.010 mm and 0.100 mm, and a depth of between 0.01 mm
and 0.1 mm.
14. The cup of claim 11 further comprising a backing having a shape
defining a point of maximum load upon the concave surface, and the
concave surface is comprised of a ceramic, wherein the debris
reservoir is a ring provided substantially concentrically about the
point of maximum load.
15. The cup of claim 14 wherein the point of maximum load defines
an angle .tau. of between 10 and 60 degrees.
16. The cup of claim 14 wherein the debris reservoir has a radius
about the point of maximum load defining an angle .theta. of no
more than 20 degrees.
17. The cup of claim 11 wherein the concave surface has i) a
centerpoint, and ii) a maximum diameter defining an equator, the
centerpoint and the equator each defining a position .beta. along
the substantially hemispherical concave surface of 0 degrees and 90
degrees respectively, and wherein at least one debris reservoir is
provided substantially concentrically about the centerpoint and has
a radius extending to the position .beta., wherein .beta. is
between about 10 and 60 degrees.
18. The cup of claim 11 wherein the concave surface has a
centerpoint and at least one debris reservoir is located
substantially near the centerpoint.
19. The cup of claim 11 further comprising i) an inner void and ii)
a channel which provides fluid connection between the inner void
and the at least one debris reservoir.
20. A femoral prosthesis comprising:
a) a substantially spherical hip joint prosthesis head having an
outer surface, and
b) an acetabular cup having a concave surface, the concave surface
having a shape substantially corresponding to the outer surface of
the head,
wherein the head outer surface and the concave surface of the cup
are positioned to substantially contact and articulate with each
other to define an articulation surface, and
wherein at least one of the outer surface of the head and the
concave surface of the cup comprises at least one debris reservoir
thereon,
wherein the at least one debris reservoir is provided on the head
outer surface, wherein the head tapered recess defines a
frustoconical surface, wherein the head further comprises a channel
in fluid connection with the at least one debris reservoir, wherein
the prosthesis further comprises:
c) a trunnion having a tapered stem adapted for friction fitting
with the tapered recess of the head, the tapered stem being
substantially friction fitted into the tapered recess of the head,
to form a reserve defined by the friction fit of the frustoconical
surface of the head and the stem,
and wherein the channel provides fluid connection between the at
least one debris reservoir and the reserve.
Description
BACKGROUND OF THE INVENTION
One of the most challenging problems in the field of artificial hip
joints is the wear debris caused by articulation between the
surfaces of the polyethylene acetabular cup and the metal or
ceramic hip joint head. Even at a low wear rate such as 0.1 mm per
year, the debris resulting from this wear is considered to be
significant because it enters the vascular system of the human body
and may cause osteolysis.
Some investigators have proposed reducing the wear problem by a
variety of different methods. For example, U.S. Pat. No. 5,378,228
("Schmalzried") suggests surrounding the prosthetic device with a
series of funnels and reservoirs. U.S. Pat. No. 5,514,184 ("Doi")
suggests providing a protector along the outer edge of the cup to
catch debris. U.S. Pat. No. 4,822,368 ("Collier"), EPO Published
Application No. 0 346 294 ("Impallomeni"), and U.S. Pat. No.
5,514,182 ("Shea") each suggest surrounding at least one of the
articulation surfaces with a semi-permeable membrane which permits
fluid circulation but traps debris. However, each of these systems
adds greatly to the complexity of the prosthesis.
Therefore, there is a need for a simple bioprosthetic system which
reduces the danger caused by articulation-induced wear.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a joint
bioprosthesis comprising:
i) a material suitable for implantation within a human body,
ii) a first surface having a shape suitable for fixation, and
iii) a second surface having a shape suitable for joint
articulation,
wherein the second surface comprises at least one debris reservoir
thereon.
In some embodiments, at least a portion of the second surface has a
surface roughness R.sub.a of no more than 50 nm. In other
embodiments, the debris reservoir has a width of between 0.010 mm
and 2 mm. In preferred embodiments, the second surface is either i)
substantially spherical (for use as a hip joint head), ii) concave
and substantially hemispherical (for use as a hip joint cup), or
iii) has a shape suitable for articulation with a third surface
selected from the group consisting of a tibial knee joint surface
and a femoral knee joint surface. In some embodiments, the first
surface is either a frustoconical recess (for fixation to a hip
trunnion), or has a shape suitable for fixation with an acetabulum.
In some embodiments, the material is preferably a biomedical grade
ceramic, and more preferably a biomedical grade zirconia.
Also in accordance with the present invention, there is provided a
hip joint prosthesis head comprising:
a) a substantially spherical outer surface having a diameter of
between 15 mm and 50 mm, and
b) a tapered recess extending inward from the outer surface, the
recess having an outer diameter of at least 4 mm and a depth of at
least 6 mm, wherein the outer surface comprises at least one debris
reservoir thereon having a width of between 0.010 mm and 2 mm.
Also in accordance with the present invention, there is provided an
acetabular cup having a substantially hemispherical concave surface
preferably having a maximum diameter of between 15 mm and 50 mm,
the concave surface comprising a material selected from the group
consisting of a metal, a polymer and a ceramic, and further
comprising at least one debris reservoir thereon having a width of
between 0.010 mm and 2 mm.
Also in accordance with the present invention, there is provided a
femoral prosthesis comprising:
a) a substantially spherical hip joint prosthesis head having an
outer surface, and
b) an acetabular cup having a concave surface, the concave surface
having a shape substantially corresponding to the outer surface of
the head,
wherein the head outer surface and the concave surface of the cup
are positioned to substantially contact and articulate with each
other to define an articulation interface, and
wherein at least one of the outer surface of the head and the
recessed surface of the cup comprises at least one debris reservoir
thereon.
DESCRIPTION OF THE FIGURES
FIG. 1 is a cross section of a conventional hip joint prosthesis
comprising a smooth surfaced head H, a trunnion T and a smooth
surfaced cup, wherein the head H is attached to the trunnion T and
its outer surface articulates with the concave surface of cup
C.
FIGS. 2a and 2b are perspective views of substantially spherical
hip joint prosthesis heads having debris reservoirs upon their
outer surfaces.
FIG. 3 is a cross sectional view of an acetabular cup having a
plurality of debris reservoirs upon its concave surface.
FIG. 4 is a cross sectional view of an articulation surface having
debris reservoirs thereon.
FIG. 5 is a cross-section of a hip joint prosthesis which
identifies the point of maximum loading ML.
FIG. 6 is a cross-section of a hip joint prosthesis which
identifies a debris reservoir 73 located substantially near the
centerpoint of the concave surface of the acetabular cup.
FIG. 7 is a cross-section of a hip joint prosthesis which
identifies debris reservoir 75C and 75D located substantially near
the equatorial region of the concave surface of the acetabular
cup.
FIG. 8 is a cross-section of a hip joint prosthesis wherein a gap
develops between the head and cup.
FIG. 9 is a cross-section of a hip joint prosthesis wherein
reservoir 77E and 77F is located substantially about the point of
maximum loading ML.
FIG. 10 is a cross-section of a hip joint prosthesis wherein
reservoir 78a is shaped as a circle having a midpoint at the point
of maximum loading ML.
FIG. 11 is a cross sectional view of a hip joint prosthesis of the
present invention comprising a head whose substantially spherical
surface has debris reservoirs thereon, and an acetabular cup whose
concave surface has debris reservoirs thereon.
DETAILED DESCRIPTION OF THE INVENTION
The problems caused by wear debris in a hip joint prosthesis can be
at least partially solved by providing at least one of the
articulation surfaces thereof with at least one debris reservoir
thereon. This debris reservoir will act as a receptacle for
collecting debris generated at the articulation interface, thereby
preventing the debris from leaving the prosthesis area and entering
the vascular system. Since the body's macrophages typically do not
have access to the sliding interface, the presence of the debris in
the grooves will not trigger an adverse reaction and so will not
cause harm to the body. In addition, removal of the debris by the
debris reservoir also acts to clean the articulation interface,
thereby preventing exacerbation of debris-induced wear at the
interface.
Preferably, the joint is a hip joint comprising a hip joint head
which articulates with a hip joint cup. Referring now to the
substantially spherical hip joint prosthesis head 10 provided in
FIG. 2a, the debris reservoir can assume any recessed shape,
including continuous channels 1, longitudinal rings 2a and 2b
around the spherical surface, isolated channels such as
hemispherical holes 5a, square holes 5b, grooves 6 radially
extending from a center hole 7, or reservoir 8 placed in fluid
connection with an inner void region 9 via channel 21. Preferably,
the reservoirs are longitudinal rings. These reservoirs can also be
advantageously used within the concave surface of the acetabular
cup as well, as shown by the groove 16 in FIG. 3.
When provided on the surface of the conventional hip joint
prosthesis, the dimensions of the debris reservoir are much smaller
than those of the larger tapered recess 20 (shown in FIG. 2a). The
tapered recess is commonly provided for tapered locking with a
femoral trunnion and is typically at least 4 mm in outer diameter
and at least 6 mm in depth.
In some general embodiments of the present invention, the
reservoirs are provided as rings substantially centered about
either the apex of the head or the centerpoint of the cup's concave
surface. In particular, in head-based embodiments, the head has an
internal centerpoint C (as shown in FIG. 2b), wherein the outer
surface of the head defines an apex A located substantially
opposite the tapered recess 20 and an axis X connecting the apex
and the centerpoint, and wherein at least one debris reservoir 2c
is provided on the outer surface at a position which defines a line
L between the centerpoint C and the position of the debris
reservoir, the line L and the axis X defining an intersection at
the centerpoint C, wherein the intersection of the axis X and the
line L defines an angle .alpha. which is typically between 10 and
60 degrees (and preferably is between 30 and 45 degrees). Lines
L.sub.30 and L.sub.45 in FIG. 2b represent lines whose intersection
with axis X produce angles .alpha. of 30 and 45 degrees,
respectively, while debris reservoir 2c lies within the angle
30.degree.<.alpha.<45.degree.. In cup-based embodiments, and
referring now to FIG. 3, the cup's concave surface has i) a
centerpoint P, and ii) a maximum diameter MD defining an equator,
the centerpoint P and the equator positioned along the
substantially hemispherical surface at an angle .beta. of 0 degrees
and 90 degrees respectively, and at least one debris reservoir is
provided at a position defining an angle .beta. of between 10 and
60 degrees (preferably between about 30 and 45 degrees). In FIG. 3,
reservoirs 20 and 16 are located at positions of about 30 and 45
degrees, respectively. Because these embodiments possess symmetry
about the apex/centerpoint of the prosthetic component, they do not
require extra care on the part of the surgeon to specifically
orient them during surgery.
When the head outer surface is ceramic, the corresponding concave
surface of the cup will be either ceramic or polymer. As the type,
amount and effect of debris produced by articulation in a polymer
cup-containing prosthesis is different than that produced by a
ceramic cup-containing prosthesis, the purpose of the debris
reservoir should also change. Accordingly, in more preferred
embodiments, desirable selection of the debris reservoir's
dimensions, location and position will vary depending upon whether
the cup's concave surface is ceramic or polymer.
The positions of the reservoirs upon the articulation surface
should be selected in accordance with the predetermined area of
maximum wear upon the articulation surface. Once implanted, the
orientation of the cup relative to the human acetabulum is such
that the maximum load upon either articulation surface occurs at an
angle .gamma. (typically, between about 30.degree. and 45.degree.,
and most typically is about 37.degree.) from each of the
centerpoint P of the cup's concave surface and the apex A of the
head (as shown by point ML and angle .gamma. in FIG. 5). In
particular, whereas the articulation surfaces positioned near point
ML experience about 100 psi load, the surfaces near the equatorial
regions C and D of the cup's concave surface and the head may
experience much lower loads. Since a region having an increased
normal loading likely has increased wear, positioning of the
reservoirs should reflect concern for this area of increased
wear.
When the cup has a polymer concave surface (such as polyethylene),
a relatively large amount of debris (most of which is polymer)
should be expected. As polymer debris is considered to be dangerous
to the body in the expected quantities, this debris should not be
allowed to leave the prosthesis. Accordingly, the reservoirs in
prostheses having polymer cups should be designed to effectively
retain any debris generated at the articulation interface.
The dimensions of the debris reservoirs in a prosthesis having a
polymer cup should be sufficiently large to retain the large amount
of polymer debris expected to be generated over the life of the
prosthesis. Reference is now made to FIG. 4, which presents debris
reservoirs as grooves 5a and 5b formed in articulation surface 30.
Each debris reservoir has a depth D and a width W. For a polymer
cup containing-prosthesis, these reservoirs should be large, that
is, between about 0.5 and 2 mm wide (W), preferably between about
0.5 and 1 mm in width. If the reservoir is narrower than 0.5 mm in
width, it may have insufficient storage volume for retaining the
expected volume of polymer debris particles. If the reservoir is
wider than 2 mm, then local stresses may result. Similarly, the
depth of a reservoir in a prosthesis having a polymer cup is also
large, preferably between about 0.5 and 5 mm deep (D), more
preferably between 1 mm and 2 mm. If the reservoir is less than
about 0.5 mm deep, it likely has insufficient storage volume for
retaining the expected volume of polymer debris particles. In some
polymer cup embodiments, the reservoirs are about 1 mm wide and
about 1 mm deep.
Generally, the debris reservoirs in a polymer cup-containing
prosthesis may be located on either the cup or head articulation
surface. For example, reservoirs may be provided on the head outer
surface essentially opposite the recess (i.e., at the apex of the
head), as shown by reservoir 8 and longitudinal rings 2a and 2b of
FIG. 2a. Preferably, the debris reservoirs are positioned anywhere
on either articulation surface which remains in constant contact
with its corresponding articulation surface. As the wide range of
motion in normal articulation frequently results in nearly every
portion of the head surface being out of the articulation interface
for some significant period of time (thereby allowing the
retained-but-dangerous polymer debris to exit the prosthesis), the
reservoirs in a polymer cup-containing prosthesis are more
preferably located on the cup's concave surface.
The "point of maximum load" as used herein to define the position
of a reservoir on a cup's concave surface varies from cup design to
cup design, but is typically an easily identifiable region within
any cup. Simply, it is the point on the concave surface at which
the concave surface, when in-place in an upright human, is normal
to the vertical plane. Because it bears a substantially normal
load, this surface will experience the highest wear of any portion
of the concave surface of the cup. This point can be easily
identified after use in in vivo or hip joint simulator tests, as it
is in the middle of the area having the most scratches, and this
area typically spans about 20 degrees in every direction from the
point of maximum load. However, the point of maximum load can also
be identified by the shape of a cup's backing portion. Typically, a
cup backing is not designed with 360 degree symmetry, but rather
includes asymmetrically-placed backing components such as pegs and
holes for fitting into the acetabulum in a desired, predetermined
angle of orientation. This angle of orientation (as defined by
backing pegs G and holes H, as shown in FIG. 5) allows the point of
maximum load to be easily identified, thereby also defining the
area of maximum wear on the cup's concave surface. Typically, the
positions of the point of maximum load ML and the cup centerpoint P
define an angle of maximum load .gamma., as shown in FIG. 5, which
is between about 10 and 60 degrees, preferably between 30 and 45
degrees.
In one embodiment, as shown in FIG. 5, the debris reservoir 71 is
located at the point of maximum load ML. The advantage of locating
a debris reservoir at this point is that this position provides the
greatest opportunity for quickly trapping polymer debris as it is
generated. However, as the reservoir is located in the region of
highest polymer wear, there is also a great danger that the depth
of the reservoir will be reduced over time (e.g., lose about 1 mm
in depth over about 10 years), thereby freeing the polymer debris
it has trapped over the years. This wear problem could be mitigated
by providing a deep hole in fluid connection with an inner void, as
shown in FIG. 3. However, this mitigation adds to the complexity of
the design. Moreover, even if the wear problem was so managed, the
reservoir (which is preferably no more than 2 mm wide) is still
essentially only a point sink, and so is not very effective in
trapping all the debris being generated everywhere on the
articulation interface, though mostly at the area of maximum
wear.
Therefore, in one preferred embodiment, shown in FIG. 6, the debris
reservoir 73 is located substantially near the centerpoint P of the
concave surface, as is hole 17 of FIG. 3. Another advantage of
locating the debris reservoir at the centerpoint of the cup is that
this location provides the least opportunity for debris to escape
the articulation interface. However, as this reservoir is
restricted to essentially a single point, it also is not very
effective in trapping all the generated polymer debris.
In a second preferred embodiment, shown in FIG. 7, a reservoir 75C
and 75D substantially traverses near the equatorial region of the
concave surface of the acetabular cup (i.e., near the recessed
surface's maximum diameter, also as shown by reservoir 11 in FIG.
3). However, these equatorial reservoirs should not be so large and
so near the cup's edge that the material near the edge of the cup
flexes (i.e., the edge of the cup should be rigid). By placing the
debris reservoir 75C and 75D as far away from the point of maximum
load ML as possible, the problem of erosion is minimized. In
addition, the large ring which is the reservoir 75C and 75D
provides a much larger debris-retention capacity than the point
sinks of FIGS. 5 and 6. However, it also is known that, during
service, the continual normal loading of the head against the
slightly-tilted polymer cup concave surface tends to asymmetrically
deform the cup surface to a shape shown by the dotted line in FIG.
7. In this condition, the head moves further into the cup than
desired and there is a danger that a gap will develop between the
head surface and the cup surface located near the portion of the
reservoir at 75D, thereby freeing the debris reservoir from the
articulation interface and allowing polymer debris trapped therein
to enter the body, as shown in FIG. 8. (Although the cup surface
may also deform at point C, that deformed region may nonetheless
continue to contact the head surface and the debris therein may
remain trapped.) In addition, since the reservoir section located
at 75C is closer to the area of maximum wear, there is a danger
that it will trap debris faster than the portion of the reservoir
at 75D, and so overflow quicker.
Therefore, in a third preferred embodiment, the problems of low
trapping volume, reservoir erosion, non-uniform trapping and cup
deformation are mitigated by providing at least one reservoir 77E
and 77F as a ring substantially about the point of maximum load ML,
as shown in FIG. 9. Because this reservoir is located relatively
far away from the point of maximum load ML, the danger of wearing
away its walls is reduced. Because the reservoir at point 77E is
located relatively far away from the equatorial region (as compared
to 75D of FIG. 7), it will still contact the head surface after
deformation, and not release debris. In addition, as the reservoir
77 is located essentially concentrically about the point of maximum
load ML, the debris will more likely enter the reservoir 77E and
77F at a substantially more spatially uniform rate.
When the cup concave surface and the head outer surface are each
ceramic, not only it is expected that ceramic-ceramic sliding will
produce much less debris (and so lessen the need for high volume
reservoirs), but also the ceramic debris produced is relatively
bioinert and therefore much less dangerous to the body.
Accordingly, the need for the reservoirs in a ceramic/ceramic
coupling to retain large amounts of debris are substantially
lessened. However, the same ceramic debris is much more dangerous
to the highly polished ceramic sliding surfaces than polyethylene
debris. Investigation of the effect of ceramic debris on polished
ceramic sliding surfaces has shown that the ceramic debris tends to
stick to one of the ceramic sliding surfaces and become
mini-grinding wheels, thereby accelerating wear. Although
lubrication may help remove this stuck debris, there may be
occasions in the lifetime of the hip joint prosthesis when the
surface has insufficient lubrication for this purpose. Therefore,
in these situations, the function of the reservoirs is primarily
that of removing the dangerous ceramic debris from the polished
sliding surfaces as quickly as possible.
As the need for retaining large amounts of debris is lessened in
the ceramic-ceramic coupling, the volume of the reservoirs is
correspondingly lessened. Accordingly, referring again to the
reservoir dimensions of FIG. 4, when the cup and head are each
ceramic, the reservoirs are preferably between about 0.010 mm and
0.100 mm, preferably between about 0.020 mm and 0.050 mm in depth.
Similarly, the reservoirs are preferably between about 0.010 mm and
0.100 mm, preferably between about 0.020 mm and 0.050 mm in width.
These smaller reservoirs have the advantage that they will likely
be much less deleterious to the strength of the cup or head surface
upon which they are placed. In addition, in these ceramic-ceramic
couplings, the reservoir preferably has rounded edges R to mitigate
catastrophic damage.
When the cup and head are each ceramic, the volume of debris
expected to be generated is much smaller, and its effects are much
less of a concern, and so there is no critical need to prevent the
debris from exiting the prosthesis. In light of this lessened need,
the location of the reservoirs need not be so restricted.
Accordingly, the reservoirs in ceramic-ceramic couplings can be
suitably located upon the head surface as well as the cup surface,
even if their location on the head surface periodically becomes
free of the articulation interface.
As noted above, when the head and cup are each ceramic, the
function of the reservoir is primarily that of removing the
dangerous ceramic debris from the polished sliding surfaces as
quickly as possible. Accordingly, the reservoirs should be
positioned to provide quick removal of the wear-accelerating
ceramic debris.
In one embodiment, a reservoir is positioned (on either
articulation surface) as a ring about the line of maximum load.
Since the amount of debris is expected to be small, both the
erosion of the reservoir and the volume of debris are likewise
small, and so reservoir overflow is not a concern. Preferably, the
reservoir 78a is shaped as a circle having an midpoint at the point
of maximum loading ML, as shown on the had in FIG. 10. On either
articulation surface, the circular reservoir is generally centered
about a point distanced from either the head apex A or the cup
centerpoint P by an angle .tau. of between 10 and 60 degrees,
preferably between 30 and 45 degrees, more preferably about
370.degree. in order to remove the ceramic debris from the sites of
its most rapid generation as quickly as possible. More preferably,
the circular reservoir has a radius which defines an angle .theta.
of no more than about 20.degree. relative to the point of maximum
load ML. In some embodiments, there are a plurality of debris
reservoirs 78a and 78b, each positioned concentrically about the
point of maximum loading ML, preferably each defining an angle
.theta. of no more than about 20.degree..
In general, when the head outer surface is metallic, the
corresponding concave surface of the cup will be either metal or
polymer.
The acetabular cup can be made of any material commonly used for an
acetabular cup, including polymers, metals and biomedical grade
ceramics. If a polymer is selected, then polyethylene is preferred.
If metals are selected, chrome-cobalt and titanium alloys are
preferred. If ceramics are selected, the cup may comprise alumina,
zirconia or mixtures thereof. In some embodiments, the cup can be
made of yttria tetragonal zirconia polycrystal (YTZP) ceramic. In
others, at least the concave surface of the cup is made of fine
grained (less than 2 um) alumina. Typically, the cup's concave
surface has a diameter of between 15 mm and 50 mm (preferably
between 22 mm and 32 mm), and a surface roughness Ra of no more
than 50 nm, preferably no more than 10 nm. Preferably, the cup has
a density of at least 99% of theoretical density.
In embodiments related to the hip joint prosthesis, the head of the
present invention can be made of any material commonly used as a
head in a hip joint prosthetic, including metals and obiomedical
grade ceramics. If ceramics are selected, the cup may comprise
alumina, zirconia or mixtures thereof. In some embodiments, the cup
can be made of yttria tetragonal zirconia polycrystal (YTZP)
ceramic. Typically, the head's outer surface has a diameter of
between 15 mm and 50 mm (preferably between 22 mm and 32 mm), and a
surface roughness Ra of no more than 10 nm, preferably no more than
5 nm. Preferably, the head has a density of at least 99% of
theoretical density, and its tapered recess has a total angle of
between 4.degree. and 10.degree., preferably about 6.degree..
When a zirconia is selected for use as an articulation surface, it
preferably consists essentially of a biomedical grade zirconia
ceramic comprising at least about 90 mol % zirconia, and more
preferably is a partially stabilized zirconia (PSZ) which contains
at least about 90% tetragonal zirconia. The PSZ is typically
partially stabilized by a rare earth oxide (which includes yttria)
at a concentration of between about 2 mol % and about 5 mol %. Most
preferably, the PSZ is yttria stabilized tetragonal zirconia
polycrystal (YTZP). Preferably, the YTZP has a mean grain size (SEM
using ASTM E 112/82) of no more than 1 micron (um), preferably
between 0.3 and 0.8 um. The bulk of the head should have a four
point flexural strength of at least about 920 MPa, preferably at
least 1300 MPa. Its density should be at least 99.7% of theoretical
density, preferably at least 99.8%. In some embodiments, it has an
elasticity modulus of no more than 220 GPa; an open porosity of no
more than 0.1%; less than 1% impurities; and a fracture toughness
(as per Chantikul) of at least 5 MPa m.sup.1/2.
In one preferred method of making the YTZP zirconia, the rare earth
oxide powder is co-precipitated with zirconia powder to produce a
powder which is cold isostatically pressed at between 50 and 400
MPa and appropriately green machined to form a green sphere which
is then sintered at between about 1300.degree. C. and 1500.degree.
C. for about 1 to 4 hours to achieve a density of at least 95%; and
the sintered piece is hipped in an inert gas such as argon at
between 1300.degree. C. and 1500.degree. C. for between 0.5 and 4
hours to produce a sintered sphere having a density of at least
99.9%, and a grain size of less than one micron.
If alumina is selected as the ceramic, it preferably has a grain
size of less than two microns, preferably less than one micron (by
linear intercept method). Preferably, this alumina ceramic has a
density of at least 3.9 g/cc, more preferably at least 3.97 g/cc;
and a grain size of between 1 and 2 um. In some embodiments, it has
a 4 point flexural strength of at least 400 MPa, more preferably at
least 550 MPa. A preferred alumina can be produced by sintering
biomedical grade alumina at about 1300.degree.-1500.degree. C. for
about 60 minutes and then hipping at 1300.degree.-1500.degree. C.
and 200 MPa for 60 minutes. In addition, sol gel processes such as
those disclosed in U.S. Ser. No. 07/884,817, now abandoned, or U.S.
Pat. No. 4,657,754, the specifications of which are incorporated by
reference, can also be used to make fine grained alumina.
Preferably, each articulation surface has a surface roughness Ra
which is as low as possible. If either a ceramic cup or a ceramic
head is used, the articulating surface thereon may be polished to
the desired surface roughness Ra in accordance with U.S. Ser. Nos.
08/609,711 and/or 08/521,152, the specifications of which are
incorporated by reference herein. Preferably, the surface roughness
Ra for such ceramic surfaces is no more than 50 nm, more preferably
no more than 10 nm.
In use, and referring now to FIG. 11, in one embodiment, there is
provided a femoral prosthesis according to the present invention
comprising:
a) a substantially spherical hip joint prosthesis head having an
outer surface, and
b) an acetabular cup having a concave surface, the concave surface
having a shape substantially corresponding to the outer surface of
the head,
wherein the head outer surface and the concave surface of the cup
are positioned to substantially contact and articulate with each
other to define an articulation surface, and
wherein at least one of the outer surface of the head and the
concave surface of the cup comprises at least one debris reservoir
thereon within the articulation surface.
The first end 53 of metal trunnion 52 is implanted into femur 51.
The second end of the trunnion 52 is shaped to a frustocone 54. The
outer surface 82 of the head 55 has debris reservoirs 81 thereon.
The recess 20 of the zirconia head 55 has about the same tape angle
as cone 54, and is press fit onto cone 54. A reserve 58 between the
frustocone 54 and the crown 66 is also shown. Concurrently, an
acetabular cup 63 having a concave surface 64 for receiving the
head 55 is fitted into the pelvic bone 65. Concave surface 64
further comprises debris reservoirs 80 for receiving debris. The
head 55 is positioned in the concave surface 64 of the acetabular
cup 63 to form the hip joint. When the hip pivots and the
articulation surfaces move relative to each other, debris particles
are generated at the articulation interface, predominantly near the
apex of the cup. These particles will be swept by the motion of the
spherical head and fall into the debris reservoirs 80 and 81 which
will retain them. If synovial fluid is present within the sliding
interface, the sweeping of the debris particles will be further
enhanced.
In other embodiments of the invention, the acetabular cup's concave
surface is provided with a lip action 15, as shown in FIG. 3.
During articulation, this lip wipes the surface of the femoral head
and sweeps the interfacial debris into the reservoirs. A lip is
typically created by attaching a second material such as a plastic
to the cup. The second material should have superior wiping
properties, including durability and flexibility. It typically has
a length of about 2 mm.
Although keeping the width and depth of the grooves small has
certain mechanical advantages, such small grooves will likely more
quickly fill up and may overflow with debris, and the prosthesis
will thereby lose the advantage of debris having been removed from
its sliding surface. In some embodiments of the present invention,
the potential overflow problem is solved by providing a narrow
internal channel which provides fluid connection between the debris
reservoirs at the surface of the prosthetic component with a more
remote reservoir (or inner void) within the component. This is
demonstrated in a head by channel 8, cylindrical hole 21, and inner
void 9 in FIG. 2a. It is also shown in the cup of FIG. 3 by channel
19 providing fluid connection between reservoir 16 and inner void
18. This narrow channel allows the debris to drain from the surface
grooves to a more secure reservoir, and thereby replenishes the
debris-carrying capacity of the reservoirs at the component
surface.
As shown in FIG. 11, in one embodiment, the remote reservoir is
reserve 58 which is in fluid connection with debris reservoir 81
via channel 58. Therefore, in some embodiments, the at least one
debris reservoir is provided on the head outer surface, the head
recess defines a frustoconical surface, the head further comprises
a channel in fluid connection with the at least one debris
reservoir, and the prosthesis further comprises:
c) a trunnion having a tapered stem adapted for friction fitting
with the tapered recess of the head, the tapered stem being
substantially friction fitted into the tapered recess of the head,
to form a reserve defined by the friction fit of the frustoconical
surface of the head and the stem,
wherein the channel provides fluid connection between the at least
one debris reservoir and the reserve.
In one embodiment of the present invention which uses the separate
reservoir concept, a small cylindrical hole is provided near the
apex of the spherical head which extends all the way to the taper
recess, and the void is formed by the reserve between the upper
portion of the trunnion and the deepest portion of the taper
recess. The variable motion of this hole across the polyethylene
cup surface would allow it to collect debris from the area of
maximum wear.
If desired, the channels can also be provided with filter membranes
which would allow fluid to drain through while retaining the
debris.
It is acknowledged by the art that there is a possibility that the
articulation surfaces of the bioprosthetic component can become dry
during use, with the result of increasing friction and wear.
However, the grooves of the present invention can also trap and
retain synovial fluid and thereby act as a source for lubrication
of the sliding surface. In use, lubricant trapped in the grooves
could be released into the articulation interface when the surfaces
surrounding the grooves slightly wear. The released lubricant could
prevent further wear by reducing the friction coefficient of the
articulation surfaces. It is believed that the embodiments of the
present invention in which the grooves are connected to interior
reservoirs by channels would provide a satisfactory means for
trapping and retaining the lubrication near the sliding
interface.
The present invention can be advantageously used within any
bioprosthetic device in which sliding wear (especially from
articulation) is expected to occur. This includes the hip joint
prosthetic, the knee prosthetic, and shoulder prosthetic.
Preferably, however, the prosthetic is a hip joint prosthetic
comprising an acetabular cup and a spherical head.
As mentioned above, the present invention can also be
advantageously used in a knee joint application. Therefore, in
accordance with the present invention, there is also provided a
knee joint prosthesis comprising:
a) a femoral component having a base and a plurality of tynes
extending therefrom in the same direction, and
b) a tibial plate having a receiving surface shaped for receiving
the tynes, wherein the receiving surface has at least debris
reservoir thereon having a width of between 0.1 mm and 2 mm.
* * * * *